The invention relates to a focusing method and apparatus therefor which is used in an optical system, and more particularly, to a focusing method and a focusing apparatus which is used in an optical system such as an endoscope which is provided with an illumination unit to permit observation of an object which is illuminated by the light from the illumination unit alone or principally illuminated by such light even though extraneous light may be present which is relatively weak.
When observing and taking a picture of an object with an endoscope, a photographic camera or a microscope, such instrument must be focused onto the object to be observed or photographed. In the prior art practice, a focusing operation takes place by moving an objective lens or the like of an optical system until a point is reached where the most clearly defined image of the object is obtained, while observing it through the system.
More recently, a double image coincidence scheme or ultrasonic focusing scheme has been proposed for the purpose of achieving an automatic focusing operation, and is it beginning to be implemented for practical use. However, such focussing schemes proposed are complex in the arrangement of apparatus required therefor, and therefore are not suitable for use in an application such as an endoscope for which an object must be observed or photographed through a narrow tube or a bundle of optical fibers.
As is well recognized, endoscopes can be categorized into a flexible type and a hard type. In addition, they are classified into medical and industrial endoscopes depending on the intended applications. In any event, each of these types of endoscopes is provided with an illumination unit to permit observation or the taking of a picture of the interior such as a coeliac cavity of a living body or the interior of a machine arrangement which has no access for external light. An object located in such internal space must be illuminated with light from the illumination unit alone.
When using an endoscope of the hard type, an objective lens contained therein may be moved while observing an object with the naked eye until a clearly defined image of the object is obtained. In such instance, an observation optical system comprises a plurality of lenses which transmit or relay an image of the object. This results in the difficulty that even when the object is not in focus, the controlling action of the eyes of the human observer permits the image to be viewed sharply, rendering it difficult to accomplish a precise focusing.
With an endoscope of the flexible type, its observation optical system includes a bundle of optical fibers in order to permit observation of a narrow and tortuous coeliac cavity such as the stomach or the bronchus of a man or a complicated interior such as an aircraft engine. A part of the bundle which defines a flexible tube adapted to be inserted into the coeliac cavity or into the interior of the machine has a reduced thickness and an increased length, so that the described focusing schemes cannot be readily applied to such applications.